The 5S DNA coding for the 5S ribosomal RNA synthesized by oocytes of Xenopus laevis (Xlo) and Xenopus borealis (Xbo) as well as those encoding the somatic 5S RNA from X. borealis (Xbs) have been purified from genomic DNA and cloned by the recombinant DNA methodology. The entire nucleotide sequence of one repeating sequence of Xlo and one cloned fragment of Xbo has been determined; about 80% of the sequence of one repeating unit of Xbs is known. A 5S DNA component has been found and cloned; it may contain the genes for X1s 5S RNA. We have developed an in vitro assay system that supports the accurate transcription of 5S RNA from 5S DNA in recombinant form. We will introduce delections and base transitions to repeating units of 5S DNA in order to identify sequences that influence accurate transcription of 5S DNA. The extract that supports 5S RNA synthesis is derived from nuclei (germinal vesicles) of Xenopus oocytes. We are trying to fractionate the active components in this reaction.